The work at BNL was financed by the US Department of Energy (DOE), Office of Basic Energy Science (DE-AC02-98CH10086). DFT calculations were performed using the computing facilities at the Center for Functional Nanomaterials, BNL. J.E. thanks INTEVEP and IDB for research grants that made possible part of this work at the Universidad Central de Venezuela. The work carried out at Seville was funded by MICINN and the Barcelona Supercomputing Center—Centro Nacional de Super-computacion (Spain). A.B.V. is on a leave of absence from the Venezuelan Institute of Scientific Investigations (IVIC).